With regard to heat balance of SOFC type fuel cell systems, one problem is a rather intensive variation that takes place in the heat generation during their life period. In the early stages of their service time, heat generation is limited. As the degradation of the stacks eventually proceeds, however, the electrical efficiency deteriorates thus leading to an increase in heat generation while the production of electricity tends to decrease.
In addition, with the development of current stack performance, the area specific resistance (ASR) of the stacks can decrease. The electrical efficiency can increase with a decreasing cost of heat generation in the stacks. In high temperature fuel cell systems, such as SOFC, there are in terms of overall heat balance notable heat losses to the surroundings due to practical mechanical and cost reasons.
In the early stages of a life time of the stacks, when their electrical efficiency lies at the top and the heat generation is at minimum, the heat energy contained in the exiting hot air flow from the stacks is not necessarily adequate to sufficiently warm up the supply air flow entering the stacks. The supply air flow is heated by a heat exchanger, so called recuperative unit, in which the secondary flow is in form of the exit flow from the cathode side.
The exiting air flow from the cathode side of the stacks is not warm enough. The temperature difference dT in the heat exchange unit for a sufficient supply air warm up would have to be set extreme small. This would result in heat exchange areas that are out of proportion, i.e., the use of oversized heat exchange units.
Once the degradation of the stacks proceeds along their service life and the heat generation increases, the heat energy contained in the exit air starts to cover up the requirement set up by the supply air preheating and the supply temperature on the cathode side is gained with more ease.
However, as a result of the impaired electric efficiency of the stacks, the amount of supply air to the cathode side needs to be raised, for example, even up to quadruple in relation to a stoichiometric burning. This is in order to maintain constant electric power generation even when the service life of the stacks is getting close to its end.
This, in turn, has as an inevitable consequence of a series of further drawbacks. First of all, an increase on the supply air flow increases the internal energy consumption for supplying the air, thus reducing output power and efficiency. This further boosts the voltage drop caused by the degradation phenomena. This, in turn, means substantial descent of the electric efficiency of the system. Since it can be desirable to maintain the total power of system as constant as possible, the compensation of the descending efficiency by increased fuel supply together with boosted supply air flow, is ensued by a considerable increase on heat generation by the stacks. In addition, in light of the increase on supply flow rate, pressure losses increase as well. This can have a negative impact on the electric efficiency of the system. An inevitable vicious circle can occur as the very end of the service life of the stacks draws closer.
To compensate for heat losses and varying heat generation, there are a few methods that can be used. For example, one method is to decrease the flow rate of the supply air flow, for example, at early stages of service life of the fuel cells. Another possibility is to increase an external reforming rate. These methods have, unfortunately, their own limitations. The air flow cannot be decreased unrestrictedly since there are rather strict limitations on the oxygen utilization rates allowed by the stacks. On the other hand, an active control of external reforming is difficult accomplish without increasing the complexity of process topology.
In addition, US 2008/0020247 A1 discloses promoting preheating of air to cathode side of the fuel cell. The system is presented schematically in FIG. 1.
The heating of the supply air 12 to the cathode side of the fuel cell unit 100 is based on utilizing the heat energy gained by the afterburner 101. For this purpose, there is arranged a special recuperator unit including mixers 2 and 4 as well as heat exchanger 3. The exit streams 5, 6 from anode and cathode side are first mixed therein in mixer 2. The evoked stream 7 is then utilized for warming up fresh cold supply air 9.
The final temperature adjustment of the supply stream 12 to the stacks 100 takes place not until after this stage. For this purpose, an air by-pass 11 is arranged. The by-pass air is mixed together with the heated air 10 in mixer 4. The adjustment of the supply temperature to the cathode side is carried out by regulating the amount of cold air 10 mixed with the warm air 11. In other words, the supply air of the cathode side is first heated up over the set target temperature and then leveled down to the set value with the help of the cold air.
This system can create problems such as increased complexity, use of an additional heat exchanger unit as well as increased demand for piping within an already cramped layout.
However, a drawback is caused by the fact that, since the temperature adjustment of the supply air on the cathode side is based on controlling the amount of extra cold air, the total mass flow of the air stream varies at a rather notable range. This makes it very difficult to control the total supply air flow entering the cathode side.
It also becomes difficult to make adjustments in favor of adapting the system to the degradation phenomena. Eventually, the above explained problems related to oversized heat exchangers and excessive piping are experienced again. In addition, the complex system and increased piping lead to further increase in pressure losses and generally inefficient functioning of the whole system.
Still another disadvantage relates to the fact that in case the afterburning stage is entirely performed prior to the recuperator unit, firstly humidity is increased, and secondly, the ascended temperature levels (for example, above 1000° C.) set high requirements on materials, i.e., the expenses for the materials would be unreasonable.